The invention relates to using photoelastic means to detect and quantify stress levels inside a sensor substrate that relate directly to external forces applied to the sensor substrate. Wesson describes in U.S. Pat. No. 4,466,295 means for using the photoelastic effect to measure stress in plates that can be related to externally applied forces. The sensor disclosed in Wesson is not particularly effective because the output signal is light intensity dependent and is adversely sensitive to environmental effects to the overall sensor system. A further drawback is that the sensor output is linear over only a narrow pressure range. Therefore, one must calibrate multiple sensors over different pressure ranges.
Sensing devices utilizing birefringent crystals have been described for temperature by Emo et al. in U.S. Pat. No. 5,255,068 entitled "Fringe Pattern Analysis of a Birefringent Modified Spectrum to Determine Environmental Temperature" which is incorporated herein by reference. Emo et al. describe an optical high temperature sensor based on a birefringent element made of a single crystal. A broad band light spectrum is transmitted through a first linear polarizer creating a linearly polarized wave. The linearly polarized wave passes through a single crystal birefringent plate at 45.degree. to the optical axis of the crystal. The polarized wave can be represented by two equal linear polarized vectors which are aligned along the optical axes. Propagation of these waves through the birefringent plate introduces a temperature dependent phase shift between the two waves. Thereafter, a second linear polarizer combines the two waves creating a modulated spectrum. Information derived from this modulated spectrum or fringe pattern is then used to measure the temperature of the birefringent plate.